1. Field of the Invention
The invention relates to a process for coordinate measurement on workpieces with a probe head that produces a signal on contact with the workpiece which is to be measured, and also a coordinate measuring device that is suitable for carrying out the process.
2. Discussion of Relevant Prior Art
A distinction is made in coordinate measurement technology between two different types of probe heads. Probe heads of the so-called switching type provide, on contact with the workpiece, either a switching signal, which is derived, e.g., from electrical switches in the bearings of the movable probe pin, or a pulse-form signal, which is produced, for example, by highly sensitive piezoelectric elements in or near the probe pin.
Switching probe heads are also known which produce two different signals in the course of a contact process: a piezo signal and in addition, a switching signal from the bearings of the deflected probe pin. The two signals are then mutually somewhat displaced in time, since the switch contact opens later than when the more sensitive piezo crystal is excited. Such probe heads are described, for example, in German Laid-Open Patent DE-OS 3,831,974 and in U.S. Pat. No. 4,177,568.
Probe heads of the measuring type permanently provide a signal proportional to the deflection of the flexible probe pin in the three coordinate directions. Such probe heads can be used not only for continuous scanning of workpieces, but also for single point measurements in which, by setting suitable trigger thresholds, a switching signal is produced from the signal that is proportional to the deflection.
In order to determine the coordinates of a contact point on a workpiece with a probe head of the switching type, the procedure has heretofore been as follows:
The counter states of the measuring systems (scale systems) of the coordinate measurement device are stored in an intermediate memory when, e.g., the intensity of the signal emitted from the piezo crystal exceeds a predetermined trigger threshold. If the second signal arrives at the machine control from the bearings within a predetermined time of, e.g., 200 ms, the contact is found to be valid, and the intermediately stored position measurement value is then transferred to the computer of the coordinate measurement value. In other cases, the control of the coordinate measuring device caused a repetition of the measurement. In this case, the control was derived from the fact that, as regards the piezo signal, the signal concerned was not one which indicates the contact with the workpiece, but for example a spurious signal originating from vibration or noise from the surroundings, the mounting, or the running of the machine. This lead to a so-called "contact optimizing" in which the machine again travels over the position in question.
A process which is very similar to that described hereinabove is also described in European Patent EP B1 0 025 485.
The known process in fact works satisfactorily in many situations. However, in a number of applications, the known process leads to invalid contacts. For example, when soft parts of aluminum or plastic are measured, or any workpieces are measured with low contact speeds, the signal of the piezoelement is absent or goes below the fault signal level. In such cases, even a repetition of the measurement gives no other results, and either the contact speed must be increased, or the electromechanical switching signal from the bearings must take over the function of the contact signal without the signal of the piezoelement. This results in deterioration of measurement accuracy, because the switching signal from the bearings occurs at a later time than the signal of the piezoelement, and thus no longer characterizes the exact contact coordinates of the contact point.
Long, thin probe pins also give rise to problems in determining the exact moment of contact. In working with these pins, only small amplitudes result from the probe signal, and the course of the probe signal is greatly scatterred.
German Patent DE 4,204,602 describes a process where the moment of contact can be more exactly determined in spite of the problems mentioned above. Here the probe signal is correlated with model signals that have been previously stored, and the exact moment of contact is determined by means of the signal course of the model signal. However, the courses of probe signals depend on a relatively large number of parameters. Many different model signals must be recorded and stored. These model signals must be preselected corresponding to the parameters that are present during the particular measurement, if the process is to provide satisfactory results.
In German Patent DE-PS 4,316,236, it is proposed to determine the exact moment of contact by approximating the rising flank of the probe signal with a suitable curve, and extrapolating this to the value U=0. It has been found that the time course of the probe signal, and in particular of the rising flank, is greatly scatterred. Depending upon the portion of the flank that is chosen for the curve fitting, completely different values result for the moment of contact extrapolated therefrom, so that this process does not operate satisfactorily.